This invention generally relates to autopilot systems for marine and other vessels, and, more particularly, to systems employing an improved sensor using a Hall-effect device.
There have been many types of sensors which have been included in systems providing detection of an angular deviation of a navigable body from a predetermined direction and automatic redirection of navigable body to the predetermined direction. These systems, generally known as autopilots, have been used extensively in both airborne and marine navigation. Airborne systems have used a gyrocompass as an angular deviation sensor. This approach has proved to be too expensive for most marine applications. Consequently, marine autopilot systems have included a variety of sensors. A first type includes those having electrical contacts disposed on a compass card, whose angular position is responsive to the earth's magnetic field, and on a rotatable course dial affixed to the compass binnacle for presetting a desired course. In a second type, angular deviation from a desired course is sensed by inductors placed on the compass card and on the course dial. In a third type, angular deviation is proportional to capacitive interaction between capacitive elements. A fourth type includes a permanent magnet located on the compass card, which interacts with an inductive device on the course dial. A fifth type indludes a light source and one or more photoelectric cells. The compass card acts as a shutter, or modulates the light beam to indicate angular deviation. A sixth type comprises flux gates, which generally include a pickup element of highly permeable magnetic material positioned adjacent the compass card.
These sensors are subject to certain errors which limit their usefulness in many regards. All of the prior sensors are sensitive to RF interference from radiotelephone units. The magnitude of this interference is increased with those sensors using alternating current signals. Those systems employing sensors of the first type, that is, the direct contact sensor, suffer from the disadvantage of physical restraint upon the compass card. Although the friction forces have been minimized by use of the "cat-whisker" approach, the sensors are still mechanical and subject to wear, contamination of contact surfaces, etc. The optical instruments are susceptible to errors resulting from changes in the transmission characteristic of a light path between the light source and photoelectric cell.
All of the previously described sensors, with the exception of those employing flux gates, suffer null errors in rough seas because the compass card has a movement in other planes besides the azimuthal or horizontal plane. Further, as the output signal under these conditions includes components proportional to the velocity and acceleration of the vessel in planes other than the azimuthal, it cannot be depended upon to obtain an indication of the rate of movement of the vessel in the azimuthal plane. As is well known, the introduction of a rate signal into an autopilot system is important for proper operation without excessive "huntint" or oscillation around a desired course direction. Because a rate signal cannot be obtained from the sensor output, a separate sensor, usually a gyrocompass or ratee sensitive gyro, must be added. The cost of a gyrocompass frequently exceeds the cost of the marine autopilot system. Therefore, other, more complicated provisions to avoid hunting are used.
The flux gate sensors, although not susceptible to errors of null arising from the movement of the vessel in planes other than the azimuthal, also cannot be used to obtain a rate signal, primarily because of hysteresis effects in the pickup unit.
These difficulties have led experimentors to search for other sensors. As detailed in a paper entitled "The Hall-effect compass" by Ross et al., Journal of Scientific Instruments, Vol. 34, pp. 479-484, Dec. 1957, the ability of a Hall-effect crystal to provide an output signal proportional to the relative orientation of the crystal and the earth's magnetic field has been known for some time.
With reference now to FIG. 1, a Hall-effect device 80 comprises a substantially rectangular block of crystalline material which is oriented with its faces parallel to arbitrary X, Y and Z axes. The crystal has dimensions X.sub.c, Y.sub.c and Z.sub.c and is provided with end electrodes 81, 82 which are coupled to a source of current I.sub.x, not shown. The upper and lower faces, respectively, of the crystal are also provided with electrodes 83, 84 which function as output terminals. If a magnetic field having an intensity H.sub.h is applied to the device 80 at an angle .theta. with respect to the X-Z plane, an output voltage V.sub.z across the terminals 83, 84 may be described as follows: ##EQU1## where, R = the Hall constant of the material y = block thickness, I.sub.x = current on x axis, H.sub.h = strength of magnetic field.
The Hall-effect device has many advantages. For example, a properly mounted Hall-effect device in a direction sensor can eliminate null errors due to movement of the compass card in planes other than azimuthal. In addition, the Hall-effect device can provide an output signal which is directly proportional to the sine of angular deviation .theta. from null. Therefore, the output signals can be differentiated to obtain a rate signal for use in autopilot systems. Also, the Hall-effect device has a sharply defined null plane which may be used for automatic direction control. With particular reference to FIG. 1, the null plane is the X-Z plane where the angle .theta. equals zero and the voltage V.sub.z is zero.
Prior systems using Hall-effect devices have not found widespread acceptance. First, the crystals used have large temperature coefficients so that temperature errors are a problem im marine applications. This problem can be overcome by the use of indium arsenide crystals which provide a compromise between the requirement for high electron mobility, and thus a relatively high value of output voltage V.sub.z, and a relatively low temperature coefficient. In the present state of art, indium arsenide material provides electron mobility ranging from 12,000 cm.sup.2 /v/sec to 20,000 cm.sup.2 /v/sec.
Second, even with use of indium arsenide crystals, the output voltage is very low. Typically, an indium arsenide crystal will produce about 0.25 millivolts per kilogauss of magnetic field intensity when the magnetic field is parallel to the magnetic axis of the crystal, that is to reference line 85 in FIG. 1. Since the earth's magnetic field has an intensity slightly less than one gauss, the amplification required for a Hall-effect device to produce a significant and useful voltage when excited only by the earth's magnetic field is so large that the signal-to-noise ratio becomes a limiting parameter.
The prior art has attempted to increase the voltage output of Hall-effect devices by using devices known as flux concentrators which increase the useable output voltage by a factor of approximately 10.sup.3. Generally, a flux concentrator comprises rods of high permeability material, such as that known by the trademarks "MUMETAL" or "PERMALLOY C" which are placed on either side of the X-Z faces of the crystal. The rods are separated therefrom by a predefined air gap and extend in a direction parallel to the magnetic axis 85.
However, the concentrator material introduces hysteresis errors when the orientation of the earth's magnetic field with respect to the concentrators is varied so that the rods become saturated. Typically, a shift in the null position will be in the order of 0.5.degree.. Since the concentrator materials are "active" magnetically, they also react with the applied magnetic field so as to form a combined field which is shifted somewhat from the earth's magnetic field. A third problem with Hall-effect devices has been their sensitivity to radio frequency interference, particularly in view of the high amplification required and their use of AC excitation.
Another problem of marine autopilot systems, not necessarily limited to those employing a Hall-effect device, has been the inability to control fast rudder movements which are needed in following seas and in cases where the vessel must be maneuvered rapidly. Present day steering systems allow rudder rates as high as 20.degree. per second. When the vessel is being run on autopilot, however, normal delays may cause over-travel of the rudder past its null or dead-band position. In many closed servo loops used in autopilot systems, "hunting" or continued oscillation of the rudder is encountered when high rates of rudder change are desired.
Some of the prior schemes for controlling hunting have included mechanical and dynamic braking of the rudder's servomotor, feedback from the servomotor to shift the null position, discharge of a condenser to reverse the motor for stopping purposes, and a widening of the dead-band or null. The first three of these approaches require elaborate control mechanisms for the servomotor, and the latter results in an undesirable loss of sensitivity at the null position. Other autopilot systems have limited the rate of rudder change when the vessel is being maneuvered under autopilot control. This approach limits the usefulness of the autopilot. In addition, it involves the use of expensive switching circuitry for electro-mechanical control systems, or the use of flow-control valves with electrically-switched bypasses for electro-hydraulic control systems. In the latter situation, an expensive two-speed servomotor is installed, with a slower speed being used for autopilot control, and a higher speed being used for manual control.
It is therefore an object of this invention to provide a magnetic direction sensor utilizing a Hall-effect device which does not require the use of a magnetically-permeable flux concentrator for the measurement of the angular deviation of a small magnetic field from a reference.
It is a further object of this invention to provide such a magnetic direction sensor which can be used as the directional compass of a ship or other moving body.
It is yet a further object of this invention to provide such a magnetic direction sensor which can be used to provide an output potential proportional to the sine of the angular rotation of a control surface or rudder of a ship or other moving body.
It is another object of this invention to provide such a magnetic direction sensor utilizing a Hall-effect device which is not sensitive to radio frequency interference.
It is yet another object of this invention to provide a combination of such a magnetic direction sensor utilizing a Hall-effect device with an autopilot system for marine vessels or the like, which system includes provisions for class I and class II servocontrol without the need for additional means for sensing the rate of rotation of the sensed magnetic field with respect to a reference.
It is still another object of this invention to provide an autopilot system wherein the steering engine is prevented from reaching a condition of instability resulting in continued oscillation or "hunting" when high rates of rudder positioning are required.